1. Field of the Disclosure
This disclosure relates to a photo mask and a method of manufacturing a liquid crystal display device of an in-plane-switching (IPS) mode using the same.
2. Description of the Related Art
In general, liquid crystal display (LCD) devices control the light transmittance of dielectric anisotropy liquid crystal using an electric field, so as to display pictures. To this end, these LCD devices each include an LCD panel configured to include a plurality of liquid crystal cells arranged in a matrix shape for the display of pictures, and a driving circuit configured to drive the LCD panel. The LCD panel is classified into an IPS mode or a vertical electric field mode, according to the direction of the electric field used for driving the liquid crystal.
An LCD device of the vertical electric field mode drives the liquid crystal using the vertical electric field between a pixel electrode and a common electrode which are respectively formed on two substrates disposed opposite to each other. As such, the vertical electric field mode LCD device has a large aperture ratio, but it has a narrow viewing angle.
On the other hand, a LCD device of the IPS mode drives the liquid crystal using the horizontal electric field between pixel and common electrodes which are disposed parallel to each other on a substrate. Accordingly, the IPS mode LCD device has a wider viewing angle than that of the vertical electric field mode LCD device.
FIG. 1 is a perspective view showing an IPS mode LCD panel of the related art. Referring to FIG. 1, the IPS mode LCD panel includes an upper substrate 11 provided with a color filter array, a lower substrate 1 provided with a thin film transistor array, and a liquid crystal molecules 18 filled between the two substrates 1 and 11.
The color filter array substrate includes a black matrix 12, color filters 14, and an overcoat layer 16 which are formed on the upper substrate 11. The black matrix 12 is formed to overlap with thin film transistors (TFTs) 6, gate lines 2, and data lines 4 on the TFT array substrate 1 given below, and define cell regions in which the color filters 14 will be formed. Such a black matrix 12 prevents light leakage and absorbs external light so that the contrast of an LCD panel increases. The color filters 14 are formed on the cell regions divided by the black matrix 12. The cell regions can be classified into red, green, and blue regions. As such, the color filters can include red, green, and blue color filter patterns formed on the respective red, green, and blue regions. The overcoat layer 16 is formed on the upper substrate 11 covered with the black matrix 12 and the color filters 14.
The TFT array substrate includes TFTs 6 formed on the lower substrate 1, pixel electrodes 8 each connected to the TFTs 6, and common electrodes 10 parallel to the pixel electrodes 8. Each of the TFTs 6 responds to a gate signal applied to its gate electrode and applies a data signal on its source electrode to the respective pixel electrode 8 via its drain electrode. To this end, the gate electrode of the TFT 6 is connected to gate line 2 transferring the gate signal, and the source electrode of the TFT 6 is connected to respective data line 4 transferring the data signal. The pixel electrode 8 is connected to the drain electrode of the respective TFT 6 and receives the data signal. The source and drain electrodes of the TFT 6 make in ohmic contact with a semiconductor pattern (not shown) which overlaps with the gate electrode in the center of a gate insulation film. The pixel electrode 8 and a finger portion of the common electrode 10 are formed parallel to each other on each of the pixel regions which are defined by crossing the gate lines 2 and the data lines 4. Each of the common electrodes 10 is connected to a respective common line 9 parallel to the gate line 2. The common electrode 10 receives a common voltage, which is used for driving the liquid crystal molecules 18, from the common lines 9.
A horizontal electric field is generated by the data signal applied to the pixel electrode 8 and the common voltage applied to the common electrode 10. The horizontal electrode field forces the liquid crystal molecules to rotate on the basis of a horizontal direction. The light transmittance of the pixel region varies along the rotated amount of the liquid crystal molecules to the horizontal direction so that a picture is displayed on the IPS mode LCD panel. The liquid crystal molecules driven by the horizontal electric field have a lower birefringence variation ratio to a viewing angle direction, in comparison with those driven by a vertical electric field. As such, the IPS mode LCD panel can improve the viewing angle.
However, the liquid crystal molecules of the IPS mode LCD panel are not uniformly driven throughout the pixel region, as shown in FIG. 2. Actually, the liquid crystal molecules disposed between the pixel electrode 8 and the common electrode 10 are normally driven by the horizontal electric field generated between the pixel electrode 8 and the common electrode 10, thereby controlling a transmission light amount. On the other hand, the molecules disposed to overlap with the pixel electrode 8 and the common electrode 10 cannot be driven. This results from the fact that the horizontal electric field is not formed in a space overlapping with the pixel and common electrodes 8 and 10. As such, the aperture ratio of the pixel region is reduced.
In order to enhance the aperture ratio in the IPS mode LCD panel, the number of effective opening regions W provided by the alternately arranged fingers of the pixel and common electrodes should increase, or the width of each effective opening region W should be enlarged. To rectify this, the fingers of the pixel and common electrodes 8 and 10 parallel to each other must have a reduced width. However, the widths of the fingers of the pixel and common electrodes 8 and 10 are limited to exposure resolution in a photolithography process.
FIGS. 3A to 3C are cross-sectional view illustrating step-by-step a electrode formation method using a photolithography process according to a related art. Referring to FIG. 3A, a conductive layer 22 is formed on a substrate 20, and a photo resist pattern 24 is formed on the conductive layer 22. The photo resist pattern 24 is prepared through exposing, developing, and baking processes. The exposing process allows a photo resist film to be partially exposed to light passing through a mask, so that a mask pattern is transcribed onto the photo resist film. In this case, the exposure resolution limit of present exposure equipment makes it difficult to form a photo resist pattern in a line width below 5 μm. As such, it is also difficult to form a conductive pattern in a line width below 4 μm.
Subsequently, the conductive layer 22 is etched through an etching process, thereby forming an electrode 26 fully covered with the photo resist pattern 25 as shown in FIG. 3B. Then, the photo resist pattern 24 is removed by a strip process, as shown in FIG. 3C. Characteristically, a wet etching process forces the conductive layer 22 to be characteristically over-etched. As such, the electrode 26 is formed to have a line width narrower than that of the photo resist pattern 24. Nevertheless, it is actually difficult to form the electrode 26 in a line width below 5 μm, even though the photo resist pattern 24 is prepared to have a minimum line width of 5 μm.
In other words, the minimum widths of the pixel and common electrodes in the IPS mode LCD panel are limited to the resolution of exposure equipment. Therefore, the aperture ratio of the IPS mode LCD panel can be enhanced above a critical value.